Variable area film soundtrack renovation

ABSTRACT

A method for reducing sound track sibilance distortion during playback of an analog optical sound track. The method comprises the steps of forming and storing an image of the analog optical sound track. Representing part of the stored image as a spatial image and applying an erosion filter to the spatial image. Converting the filtered spatial image to a signal representative of the sound track.

This application claims the benefit under 35 U.S.C. § 365 ofInternational Application PCT/US02/27597, filed Aug. 30, 2002, whichclaims the benefit of U.S. Provisional Application No. 60/322,700, filedSep. 17, 2001.

This invention relates to the reproduction of optically recorded analogsound tracks and in particular to the restoration of recorded signalquality.

BACKGROUND

Optical recording is most common format employed for analog motionpicture sound tracks. This analog format uses a variable area methodwhere illumination from a calibrated light source is passed through ashutter modulated with the audio signal. The shutter opens in proportionto the intensity or level of the audio signal and results in theillumination beam from the light source being modulated in width. Thisvarying width illumination is directed to expose a monochromaticphotographic film which when processed, for example, results in a blackaudio waveform envelope surrounded at the waveform extremities by asubstantially clear or colored film base material. In this way theinstantaneous audio signal amplitude is represented by the width of theexposed and developed film track. FIG. 1 depicts in greatly simplifiedform an arrangement for recording a variable width analog audio soundtrack.

A second method can be employed for analog motion picture soundtrackswhere the audio signal causes the total width of the photographic audiotrack to be variably exposed. In this method, termed variable density,the exposure of the complete track width is varied in accordance withthe intensity of the audio signal to produce a track which variestransmission, for example, between substantially clear or colored basefilm material and low transmission or high density areas of exposed anddeveloped photographic material. Thus the instantaneous audio signalamplitude is represented by a variation in the transmission ofillumination though the exposed and developed film track width.

Hence with either variable density or variable area recording methodsthe audio modulation (sound) can be recovered by suitably gathering, forexample by means of a photo detector, illumination transmitted throughthe sound track area.

These analog film sound recording techniques can be subject toimperfections, physical damage and contamination during recording,printing and subsequent handling. Since these recording techniques usephotographic film, the amount of light used in recording (Density) andthe exposure time (Exposure) are critical parameters. The correctdensity for recording can be determined by a series of tests todetermine the highest possible contrast whilst maintaining a minimizedimage spread distortion.

Image spread distortion results when a spurious fringing image isproduced beyond the outline of the wanted image. Typically image spreaddistortion results from diffusion of tight within the film base, betweenthe halide grains and the surrounding gelatin. This scattering of lightcauses an image to be formed just beyond the exposed area. Optimalnegative and positive density and exposure can yield a clean sharp welldefined image. However, with variable area recorded negatives, imagespreading causes the peaks of the audio modulation envelope appear to berounded while the valleys of the envelope appear to be sharpened anddecreased in width. This image distortion causes a non-symmetricalenvelope distortion which translates into both odd harmonic distortionand cross modulation distortion in the recovered audio. As the recordingdensity is increased the image spreading increases and consequentlybecomes evident as sibilance, initially in the higher frequency content,because of the shorter recorded wavelengths. Increasing the recordingdensity further, causes the distortion to become noticeable atprogressively lower frequencies in the recorded spectrum.

Sound recording film is generally only sensitive blue illumination andemploys a gray anti-halation dye to substantially reduce or eliminatehalation effects. Halation can result from reflections from the back ofthe film base causing a secondary, unwanted exposure of the emulsion.Typically a fine grain and high contrast emulsion is used with a controlgamma between 3.0 and 3.2.

The frequency response of these recording methods is determined byvarious parameters, for example, the speed at which the shutters openand close, the exposure of the film, and the modulation transferfunction MTF of the film which is directly related to light diffusion.The higher the exposure time the lower the frequency bandwidth of therecording.

With these optical recording methods the resulting audio signal to noiseratio can be optimized by use of a high contrast image. For example, thedarker audio envelope waveshape and the clearer the surroundings, thecleaner or quieter will be the sound. However, there is a limitation tothe possible density at which the film can be exposed at withoutintroducing audio distortion due to image spreading in the filmemulsion.

Optimum density presents a compromise between signal to noise ratio andimage spread distortion. An optimum density can be determined by testexposures to find an acceptably low value for cross modulationdistortion resulting from image spreading. Frequently older or archivalaudio tracks are improperly recorded and can exhibit severe distortion.However, often some image spread distortion is tolerated in order toobtain an improved audio signal to noise ratio. FIG. 2 shows a somewhatcomplementary variation of cross modulation distortion with density whenprinting from negative to positive film sound stock.

In addition to density and image spread distortion other imperfectionscan result, for example the density of the exposed or unexposed areascan vary randomly or in sections across or along the sound track area.During audio track playback such density variations can directlytranslate into spurious noise components interspersed with the wantedaudio signal.

A further source of audio track degradation relates to mechanicalimperfections variously imparted to the film and or it's reproduction.One such deficiency causes the film, or tracks thereon, to weave or movelaterally with respect to a fixed transducer. Film weave can result invarious forms of imperfection such as amplitude and phase modulation ofthe reproduced audio signal.

Analog optical recording methods are inherently susceptible to physicaldamage and contamination during handling. For example, dirt or dust canintroduce transient, random noise events. Similarly scratches in eitherthe exposed or unexposed areas can alter the optical transmissionproperties of the sound track and cause sever transient noise spikes.Furthermore other physical or mechanical consequences, such as the filmperforation, improper film path lacing or related film damage canintroduce unwanted cyclical repetitive effects into the soundtrack.These cyclical variations can introduce spurious illumination and giverise to a low frequency buzz, for example having an approximately 96 Hzrectangular pulse waveform, rich in harmonics and interspersed with thewanted audio signal. Similarly picture area light leakage into the soundtrack area can also cause image related audio degradation.

A German application DE 197 29 201 A1 discloses a telecine which scansoptically recorded sound tracks. The disclosed apparatus scans the soundinformation signal and applies two dimensional filtering to the outputvalues. A further German application DE 197 33 528 A1 describes a systemfor stereo sound signals. An evaluation circuit is utilized to provideonly the left or the right sound signal or the sum signal of both as amonophonic output signal.

Clearly an arrangement is needed that allows optically recorded analogaudio sound tracks to reproduced and processed to not only substantiallyeliminate the noted deficiencies but to enhance the quality of thereproduced audio signal.

SUMMARY OF THE INVENTION

Optical sound tracks recorded with sound signals represented by thevariable width method are subject to distortion resulting from imagespreading during the photographic process. An inventive method reducessound track sibilance distortion during playback of an analog opticalsound track. The method comprises the steps of forming and storing animage of the analog optical sound track. Representing part of the storedimage as a spatial image and applying an erosion filter to the spatialimage. Converting the filtered spatial image to a signal representativeof the sound track.

In a further inventive arrangement a selectable amount of image spreadreduction is applied to a spatial image representation of the soundtrack and the result of reducing the image spreading is assessedacoustically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an audio soundtrack using avariable area recording method.

FIG. 2 shows relationships between cross-modulation distortion andrecording density.

FIG. 3 is a block diagram of an inventive arrangement for processingoptically recorded analog audio sound tracks.

FIGS. 4A and 4B show a 16 mm film gauge implementation of the inventivearrangement of FIG. 3.

FIG. 5 shows a scanned gray scale analog image of a variable area audiosoundtrack subject to certain deficiencies.

FIG. 6 illustrates a control panel used in accordance with the inventivearrangement of FIG. 3.

FIG. 7 shows a processed scanned image of an audio soundtrack inaccordance with a further inventive arrangement.

FIG. 8A illustrates diagrammatic representations of an exemplaryelliptical area of the track image shown in FIG. 7.

FIG. 8B illustrates the result of a erosion filter processing inaccordance with a further inventive arrangement.

FIGS. 9A and 9B are charts representing sequences associated withvarious inventive arrangements.

FIGS. 10A and 10B are diagrams representing a sound track envelopereproduced with an azimuth error in FIG. 10A and corrected in FIG. 10A.

DETAILED DESCRIPTION

The block diagram of FIG. 3 shows an inventive arrangement forreproducing and processing an optically recorded analog audio soundtrack. Typically a light source 10 is projected onto film 20 whichincludes an audio sound track 25, depicted in FIG. 3 with an exaggeratedwidth dimension. The audio signal my be represented as suggested intrack 25 by means of a variable area recording method, however the audiosignal may also be represented by corresponding variations in densitysubstantially across the width of the sound track area. In aconventional film sound reproducer light from source 10 is transmittedthrough film 20 and track 25 in accordance with the method employed forexposing the sound track. However, the resulting varying intensitylight, modulated by the soundtrack, is gathered by a photo sensor suchas a photo cell or solid state photo detector. The photo sensor usuallygenerates a current or voltage in accordance with the intensity of thetransmitted tight. An analog audio output signal results from the photosensor and this is generally amplified and often processed to alter thefrequency content to improve or mitigate deficiencies in the acousticproperties of the recorded track. However, such frequency responsemanipulation, is generally incapable of remedying the deficiencieswithout adversely effecting the wanted audio content.

In the inventive arrangement shown in FIG. 3, tight from source 10 isguided by a fiber optic means (not illustrated) to from a projected beamof light for illuminating sound track 25. The light is modulated inintensity by the sound track and is collected by optical group 75.Optical group 75 includes a lens assembly, extension tube and bellowswhich are arranged to form an image of the complete sound track widthacross the width of a CCD line array sensor 110 which forms part ofcamera 100. Camera 100, for example a Basler type L160, is controlled byframe grabber 200, for example, Matrox Meteor II LVDS digital boardwhich synchronizes the image capture and outputting of an 8 bit digitalsignal representing the line scanned image of sound track 25 as the filmmoves continuously through the projected beam of light. The CCD linearray sensor 110 has 2048 pixels and provides a parallel digital outputsignal 120, quantized to 8 bits and capable of operating with a bit ratein the order of 60 MHz.

The digital image signal 120 represents successive measurements acrossthe width of the sound track which are captured as an 8 bit gray scalesignal representing the instantaneous widths of exposed and unexposedareas of the sound track. This continuos succession of track widthimages or measurements are stored by an exemplary RAID system 300 as acontinuous digital image of the optical track.

An operating system can be resident in controller 400 or as depicted byblock 405 which provides the user with a visual menu and control panelpresentation on display 500. Controller 400 can a personal computer orcan be implemented as a custom processor integrated circuit. However,the computer controller must support the high transfer rates associatedwith the camera data and requires at least 512 MB of RAM together withan Ultra SCSI 160 interface that can sustain the high transfer rates. Inaddition a dual processor computer can allow parallel processing whichcan increase both processing speed and performance.

Camera 100 has a line array CCD sensor with 2048 pixels and provides an8 bit parallel digital output signal, 120, in accordance with LVDS or RS622 output signal formats. The use of a 2048 pixel line array sensorprovides sufficient resolution to capture the soundtrack envelope imagewithout significant frequency response distortion. In addition thecamera can be controlled by a frame grabber 200, which in additionprovides synchronization to NTSC or HD television sync pulses via syncinterface 250, and also permits an output data rate sufficient tocapture sound track images at normal operating speeds of nominally 24fps.

Thus under control of frame grabber 200 and responsive to user controlfrom display and keyboard 600 the digital image is transmitted via aframe capture card 200 for storage on a hard disk memory array 300. Forexample the scanning rates employed in this advantageous arrangementresult in an exemplary file size in the order of 4 giga bytes per minuteand this bitstream is supplied for storage by a striped raid system 300which facilitates storage of the large sound track image video filewhile providing rapid transfer rates.

The optical system, bellows extension tube and lens 75 are accuratelypositioned to image the standardized recorded track positions, howevermanual adjustments are provided to permit both focusing, exposure andimage size adjustment or zoom control to allow the recorded film area tosubstantially fill the maximum sensor width with peak audio modulation.The camera mounting system also facilitates both lateral and azimuthadjustments. Lateral adjustment L allows laterally mis-positioned tracksto be imaged, for example to eliminate sprocket or perforation generatedbuzz or picture related light spill. Furthermore in severe situationswhere lateral image adjustment fails to eliminate audible sprocket holeor perforation noise, or picture spill, the camera and tens can beadjusted to substantially fill the sensor width with a part of therecorded envelope positioned to avoid the offending illuminating noisesource.

The selection of tens and optical system requirements are determinedlargely by the 35 mm audio optical track width and the width of theimager array. A 35 mm optical track has a standardized width of 2.13 mm,and the approximate length of the imager is about 20.48 mm based on apixel size of 10 microns. Thus to enable the maximum width of a 35 mmsound track to fill the imager width requires an image magnification ofabout 10:1. Similarly for a 16 mm track having a width of 1.83 mm, inorder to fill the sensor width requires the addition of a 56 mmextension tube or bellows.

In addition to the imaging considerations, the desired bandwidth of theprocessed audio signal must be considered. For example, if a reproducedaudio bandwidth of 15 kHz is required, a sampling or image scanning rateof 30 kHz is needed. Thus with an exemplary sampling rate of 30 kHz, thecamera will output 2048 bytes or 8 bit words for each image scan (audiotrack line scan) producing an output data rate of 2048*30*10³ or 61.4mega bytes per second. Hence one minute of sound track requiresapproximately 3.68 giga bytes of storage. Such storage capacityrequirements can be provided by an exemplary striped raid system such asan Ultra Wide SCSI 160 drive.

FIG. 4A illustrates an exemplary magnetic film transport manufactured byMagna-tech Electronic Co. Inc. which forms the basis for the inventivescanning arrangement and provides a servo controlled film transportsystem with adequate room for mounting the line array CCD camera. Amajor requirement is that of good film guidance and the provision of asteady film path to prevent variation of film focus as it travelsbetween the light source and camera. Through experimentation it wasdiscovered that optimum film stability for scanning was achieved at alocation where the film wraps around a flywheel. Although film imagesurface is curved at the flywheel the use of line array scanner lookingorthogonally and without azimuth errors at the film obviates problems ofdepth of field and sound track inter-modulation, and phasing or flangingdistortions.

An exemplary flywheel is depicted with a 16 mm gauge film in FIG. 4Btogether with a cranked fiber optic light guide which facilitatesorthogonal illumination of the film without obscuration by the cut awayflywheel center. In an alternative arrangement, illustrated in FIG. 4C,an exemplary flywheel provides support for a major part of the filmwidth and obviates the requirement for the cranked light guide shown inFIG. 4B. In this arrangement the 16 mm gauge film is supported by theflywheel over the majority of the film width with the exception of anominally 3 millimeter edge region which contains the sound track ortracks. Similarly when operating with 35 mm gauge film an edge region ofabout 8 mm containing the sound modulation extends beyond the exemplaryflywheel of FIG. 4C. The wrapping action of the film around the flywheelforms a partially cylindrical structure (CS) which provides rigidity andsignificant stiffness and hence resistance to edge deviation or fluttereffects. In this way the advantageous wrapped positioning of the soundtrack area relative to the flywheel ensures a stable film edge anddefocusing of the image is largely precluded.

The inventive film sound processing system is activated by keyboard 600or mouse selection of an icon (Digital AIR) which results in a Windows®like control screen arrangement presented on display screen 500. Variousoperating modes such as Preview, Record, Stop, Process and Export arepresented as tool bar functions in a border area of the display.Initially the Preview mode can be selected from the tool bar functionswhich advantageously starts the sound track in motion and forms a soundtrack image on display screen 500. The gray scale image allows alignmentof camera and optics to the recorded sound track. Optical group 75 isadjusted to ensure that peaks of the sound track image substantiallyfill the imager 110 width and to provide good image signal to noiseratio by ensuring proper CCD exposure which can differ between negativeand positive prints and is also dependent on the type of film stock.

Advantageously the real time mage provides not only pictures of thesound track but also shows the presence of interference generatingillumination emanating from the sprocket holes, or the picture areawhich can contaminate the sound track. This unwanted light ingress canbe eliminated by using the on screen camera image to permit manipulationof optical group 75 to remove such unwanted audio contributions bycarefully framing the soundtrack using picture zoom, pan and tilt. Inaddition the sound track image can be examined in detail byelectronically magnifying selectable sections of the display envelope topermit camera azimuth alignment when reproducing a test film known as abuzz track. The magnified image is presented with an electronicallycursor line which permits the evaluation of any time or phase differencebetween peaks in the modulation envelope. With optimized azimuthalignment modulation peaks appear concurrently with substantially equalmagnitude but opposite polarity. An optimum azimuth adjustment willproduce concurrently maximized envelope peaks. Misalignment of azimuthbetween the camera an the sound track can result in an image whichcaptures temporally different audio information, such as can occur witha stereo audio track pair. FIG. 10A is diagram representing a soundtrack is envelope reproduced with an exemplary and exaggerated azimutherror. Shown on the same time axis of FIG. 10A is a processed orelectronically cored image showing the temporal displacement resultingfrom an azimuth error between the camera imager camera and the soundtrack. FIG. 10B is the same envelope image as FIG. 10A but reproducedwithout an azimuth error, and shown below on the same time axis is theelectronically cored image which indicates that the envelope peaks havebeen scanned substantially concurrently and are of similar amplitudes.

An example of a Preview mode sound track image is shown in FIG. 5. Thegray scale picture in FIG. 5 is of a duplicate negative sound trackwhich includes various impairments. For example, on the right side ofthe sound track image unwanted illumination can be seen emanating fromfilm perforations, a defect indicative of misalignment duringduplication. In addition the sound track has a reduced width and showslateral scratches probably incurred on the original negative. Hence theadvantageous real time sound track image permits rapid visual alignmentof the camera and optics, rather that reliance on acousticallydetermined positioning. The scanning alignment sequence is depicted inthe sequence chart of FIG. 9A. The sound track image facilitates thesubstantial elimination of deficiencies resulting from priormisalignment. Following camera image optimization, framing, focus,exposure, etc., the Record mode is selected from the tool bar and thesound track is scanned, digitized as exemplary 8 bit words and stored inmemory 300. Upon completing the scanning and storage steps the digitalsound track image is processed by selecting the Processing mode from thetool bar.

The processing control panel shown in FIG. 6 allows the operator toselect and optimize film specific processing to be performed on thestored sound track image thereby obviating the potential for damagingthe film material during repeated play back for optimization.Advantageous processing algorithms resident, for example in controller400 or as depicted within block 410 are selected from the on screen menuvia keyboard 600 and applied to data selectively retrieved from thestored digital image in system 300. The algorithms employed to remedycertain sound track deficiencies will be explained, however, thecorrective processing sequence is depicted in the chart of FIG. 9B. Theprocessed and renovated digital signal is converted for outputting asdigital audio signal 450 with selectable exemplary formats such as WAV,MOD, DAT, DA-88.

Having stored the complete soundtrack as a digital image the inventiveProcessing mode is selected from the on screen tool bar. The processingcontrol panel shown in FIG. 6 allows the operator to select and optimizeprocessing specific to the stored sound track image. For example filmgauge is selectable, together with the film type, positive or negativeand audio modulation method for example, unilateral variable area,bilateral variable area, dual bilateral variable area, stereo variablearea or variable density. The advantageous processing algorithms areselected from the on screen menu and applied to the stored digital imageaccessed from storage system 300 for processing by the CPU or a DSP cardof controller 400.

Sound track deficiencies can result from the various causes describedpreviously. However, more specifically, dirt, debris, transverse ordiagonal scratches or longitudinal cinches in a negative can producewhite spots when printed. These flaws generate clicks and crackles. Suchwhite spots tend to affect the dark areas of the track and are morenoticeable during quiet passages whereas noise occurring during loudpassages often originates in the clear areas of the print. Low frequencythuds or pops often result from relatively large holes or spots in apositive soundtrack formed as a consequence processing problems. Hisscan result from a grainy or slightly fogged track area. Sibilance yieldsspitting S sounds and is particularly objectionable. Typically sibilanceresults from image spreading within the photographic emulsion ofvariable area recordings and gives rise to cross modulation distortionof audio signals recorded on the track.

Although the scanned audio track is represented as a continuous envelopeimage it was advantageously recognized that sections of the envelopeimage can be read from memory 300 and configured in RAM for processingusing spatial image techniques. An first algorithm was developed usingMatlab® to facilitate loading the audio envelope image as matrix ofvalues to permit the use of spatial image processing. By gathering smallconsecutive pieces of the audio envelope to form spatial image sectionsit is possible with a second algorithm to identify and eliminateextraneous pixels that differ from surrounding pixels. Withoutprocessing, such extraneous pixels can produce transient noise in thereproduced audio signal. In this second algorithm a small mask or windowcomprising, for example, 3×3 pixels is formed with groups of threepixels values from three adjacent line scans. This window is moved orstepped across the spatially configured sound track image data with thepixel of interest, or subject pixel centered in the window. If the valueof the subject pixel differs from the value of the surrounding pixels itis replaced with the value of the surrounding pixels. Thus thisalgorithm is suited to use with signals that have been subject digitalthreshold processing, which will be described, where isolated, contrarydata values can in general be associated with erroneous and ultimatelyaudio noise generating consequences. Hence such contrary data values arereplaced by the predominate value within the window. Thus each pixel ofthe scanned audio track is tested and replaced to form a processedsoundtrack image in RAM. In edge areas padding is applied to preventerroneous pixel replacement.

Scratches across sound track can produce transient or impulsive noiseeffects such as loud pops or clicks. The simple rule of pixelreplacement described in the second algorithm is less effective withcontiguous contrary value pixels. However, this form of transient noiseis advantageously eliminated by a third algorithm which is applied tospatially configured track image sections of the stored exemplary 8 bitdigital envelope signal. This third algorithm uses a further spatialimage processing technique to derive median values for each pixel ofeach image section across the width of the track. These median valuesare then used to replace the scanned image data across the track area.The median filter is implemented by an exemplary mask or windowcomprising, for example 9×9 pixels, which is progressively stepped,pixel by pixel across a spatial representation of the audio envelopedata. The center of the window represents the pixel to be corrected. Thepixel values of the track image positioned under the window are sortedor ranked in amplitude order. The middle value of the rank ordered setis then substituted for the actual track image value of the center pixelof interest, this process is then repeated for the next pixel across thewidth of the spatially configured track image. Ultimately every pixelrepresenting the scanned audio track is evaluated and if necessaryreplaced forming a processed soundtrack image in RAM.

Other mask or window sizes and shapes can be advantageously employed tofavor formation of median values. For example a 3×6 mask formed fromthree successive image scans across the sound track width will form apixel neighborhood that favors the track width in the formation of themedian value. Alternatively the mask or window can be advantageouslyfavor formation of a median value from a pixel neighborhood extendingover a greater number of successive scans but occupying less track widthfor example by use of a 9×3 mask. In addition exemplary masks can beconstructed to provide diagonal weighted image processing.

Because the median filter window analyzes data from pixel groups, withsome occurring in adjacent line scans, an amount of blurring or datasmoothing can result because the middle value of the rank ordered setcan be representative of a data value occurring at a different spatialand or temporal scanned location. However, this smoothing effect can becompensated with a two dimensional high pass filter which can sharpen orsubstantially restore the image. The median filter process iscomputationally intense and therefore time consuming but can beoptimized by recognizing that certain values within the window will notchange from step to step.

Following median filtering of the audio envelope image data whichremoves aberrant values a further operation is performed termedContrast. The Contrast process advantageously recognizes that thevariable area recording method employs only two states, one to representthe audio envelope, the second to represent the envelope's absence. Thusthe sound track has some areas that are substantially clear and othersthat are opaque. Advantageously processing screen FIG. 6 allows sectionsof the stored image to be previewed, by selecting button A, and viewingthe resulting image as contrast slider B is varied. Contrast slider Ballows a threshold value of a further software algorithm, or hardwareimplementation to be varied about a nominal center range decimal valueof 127 for an exemplary 8 bit range of image values scanned from thesound track. The algorithm classifies the pixels according to theirintensity value and splits the range of values in two. Thus for imagesdigitized with values less than the selectably adjusted threshold theactual scanned digital value, or median filtered value, is replaced witha new low digital value, for example representing decimal 0, andsubstantially equal to black or zero film transmission. Similarly fordigitized images values greater than the adjusted threshold value theactual value is replaced with a new high value substantially equal towhite or decimal value 255. In this way grayscale variations in thenominally clear and opaque film areas are removed and defects causingvariable light transmission through the track are eliminated. Thisdigital thresholding or binarization method re-quantizes the storeddigital audio envelope image into 2 states, represented by one bit.However, although contrast slider B offers the visually apparent abilityto remove or eliminate dirt, scratches and artifacts from the on screenpreview image, the result must be balanced, and acoustically judgedagainst any consequential, unintentional and unwanted changes to theaudio content.

Vertical slider bar C provides access to 10 sections of the recordedimage data, assigned on the basis of file duration, number of frames orrunning time. These 10 sound track sections allow the effect ofdiffering digital threshold values, determined by contrast slider B, tobe evaluated on track areas containing both loud and quiet passages. Theadvantageous digital thresholding or binarization process improves thesignal to noise ratio of the image signal and aids in the identificationof the edges of envelope image. FIG. 7 shows a section of a soundtrackimage subject to digital threshold processing.

Image spread distortion effects variable area recordings and results inobjectionable audio sibilance. Image spread distortion results duringrecording from scattering of light causing the growth of the image orfringe beyond the actual image outline. Since the spreading is exposuredependent the effect is initially evident in higher frequency or shorterwavelength audio content. Image spreading causes peaks of the audiomodulation envelope to become rounded while the valleys of modulationenvelope appear to be sharpened. Thus the sound image envelope becomesnon-symmetrical and causes harmonic distortion and cross modulation ofthe audio content.

Once again spatial image processing techniques are advantageous used tosignificantly reduce or substantially eliminate sound track impairmentdue to image spread distortion. Various spatial image processingalgorithms can be used to remove the envelope asymmetry caused by imagespreading. In a exemplary algorithm Sobel filters can be used to findthe outline of the audio envelope which is then further processed toidentify valleys and peaks. In accordance with the slope and amplitudeof the envelope, a weighted number of pixels are added to the envelopeimage and operational control can be provided a graphic user interfaceto control the weights of the corrective additions.

In a fourth advantageous arrangement morphological erosion filtering isemployed to significantly reduce or eliminate the effect of image spreaddistortion of the audio track envelope. Erosion filtering is performedby analyzing each pixel of the spatially configured envelope image,usually in binary or thresholded form, with a structuring element, forexample a 3×3 array having values of either one or zero. The structuringelement is stepped over each pixel of the envelope spatial image withthe center of the element covering the input pixel of interest. If thestructuring element is an 3×3 array of ones then the output value of thepixel of interest is determined by the correspondence of the envelopepixel neighborhood surrounding the pixel of interest under the array,with the values in the array. If all the neighborhood pixels and thepixel of interest match the exemplary 3×3 array of ones, then the outputvalue of the pixel of interest is not changed. However, as soon as anypart of the 3×3 array straddles an edge in the exemplary thresholdedenvelope image, the pixel of interest is changed from a one to a zero.Thus with the exemplary 3×3 structuring element an envelope edge betweenwhite and black is detected by a leading one of the neighborhood pixelscausing the adjacent center pixel, or pixel of interest, to assume thesame value as the leading neighborhood pixel, thereby causing the whiteto black transition to move, shrink or erode into the white or binaryone area.

With the exemplary 3×3 structuring element edges of the audio envelopeare eroded by one pixel. The amount of image spreading can exceed thewidth of one pixel, however a second pass of the erosion filter willremove a second pixel but at the expense of processing time. In afurther advantageous arrangement varying amounts of image spreadcorrection can be selected, as indicated in area D of FIG. 6, with thedesired degree of correction performed in a single processing step.Greater amounts of erosion can be achieved by use of a largerstructuring element, for example with a 5×5 array, erosion of two pixelsis achieved corresponding to the selectable correction of a mediumdegree of distortion. Similarly processing with a 7×7 structuringelement erodes three pixels and represents the correction of severdistortion.

Morphological erosion filtering can be performed with a softwarealgorithm, for example developed using Matlab®, or alternatively thefilter function may be implemented with hard wired logic. Howeverimplemented, the representation of the audio track envelope in thespatial domain permits the advantageous use of erosion filteringtechniques to mitigate image spread distortion, largely eliminate crossmodulation and restore the audio track fidelity.

FIG. 8A is a diagrammatic representations of exemplary elliptical area 8of the threshold processed track image depicted in FIG. 7 and shows bothwhite squares representing pixels or digital sample values and graysquares representing pixels or digital sample values from the blackareas of FIG. 7. FIG. 8A includes a representation of exemplary 3×3structuring element SE which is formed as follows,

0 1(A) 0

0 1(X) 0 0 1(B) 0having one values or active cells, A, X and B in the center column, withthe pixel of interest marked with an (X). The structuring element isstepped across the spatial representation of the track image, pixel bypixel as indicated by the arrow. Because this structuring element hasonly three active cells, the processed value of center pixel X isdetermined by the laterally adjacent pixel neighborhood as shown, wherethe center value X is determined by the following erosion algorithm,if (X·A·{overscore (B)})+(X·Ā·B)then,_X′={overscore (X)}else,_X′=X

-   ·=AND,-   +=OR,-   {overscore ( )}=NOT,-   X′=pixel in resulting image at the same position.    With this exemplary structuring element the output value of the    pixel of interest is determined by the correspondence of the track    image pixel neighborhood adjacent to the pixel of interest under the    structuring element. If the adjacent neighborhood pixels and the    pixel of interest match the structuring element, then the output    value of the pixel of interest X′ is not changed. However if either    track image values under cells A or B fail to match then the pixel    of interest X′ is changed to the complementary value, for example    zero.

The enlarged processed track image of FIG. 8A shows the advantageousstructuring element SE positioned to perform erosion filtering with FIG.8B showing the resulting eroded image where eroded pixels are shown aswhite blocks with broken outlines with the current pixel of interestdepicted with by a * symbol. The solid white squares that representedpixel values in FIG. 8A are omitted from FIG. 8B to allow the erodedpixels greater visibility.

Following the advantageous use of spatial image processing techniquesthe processed envelope image is converted back to sound signal by afurther advantageous algorithm. The conversion algorithm sums the numberof black pixels, for a negative track, or white pixels for a print, thatrepresent the audio envelope for each line scan. This number of activepixels, representing the instantaneous amplitude which is thensubtracted from the maximum amplitude value, for example 2048, whichrepresents the total sensor pixel count. The resulting differencerepresents the instantaneous audio amplitude. Clearly the converseprocess is also possible where a nominally smaller number ofnon-envelope representative end pixels are counted and subtracted fromthe total sensor pixel count with the result representing theinstantaneous audio amplitude. This audio amplitude value is then scaledto an appropriate audio signal format range. For example, using a 16 bitWAV file format the renovated audio values are scaled to fit a range of−32767 to +32768, where 0 represents DC. This audio conversion algorithmwas developed using a Matlab® image processing toolbox. The Algorithmalso includes a routine that prepares header appropriate for the fileformat and provides a streaming buffer to receive the WAV data followingconversion. In addition to WAV formatted files a variety of other audiofile formats are available including AIFF, MOD, DAT, DA-88 and DA-98HR.

In a further inventive aspect film weave which causes the sound track tovary in position relative to the audio transducer is advantageouslycorrected. The effects of film weave can appear as various types ofmodulation of the audio signal. Often an amplitude modulation resultswhere the modulation is representative of the rate of film weave. Insevere cases the reproduced audio signal can be subject to a low passfiltering effect where the cut off frequency is modulated by the filmweave. In accordance with the inventive arrangement the presence of filmweave results in the instantaneous audio envelope image also weaving ormeandering on the sensor, however, this positional image variation onlyresults in a variation of the pixels representing an envelope imageabsence. For example, in a negative track these pixels would represent aclear or high transmission part of the track and are positioned at theend regions of the array.

During the initial camera alignment the track image is observed atseveral film locations and if film weave is apparent the image centeringcan be adjusted to position the nominal center of wandering sound trackpath in the middle of the display image. The image size is then adjustedsuch that audio envelop peaks occurring at the maximum excursions of thetrack wander do not exceed the width of the CCD line array. Thus havingcentered the wandering envelope image the numbers of pixels at each endof the array are substantially similar for the centered track. Hence itcan be appreciated that as the film weaves only the numbers, ordistribution of the end (non envelope) pixels vary. However, theenvelope pixel count, which represents the envelope amplitude, remainssubstantially constant because the envelope image moved, but remained onthe sensor array. Thus the algorithm for converting the envelope imageinto an audio value advantageously eliminates and corrects the effectsof film weave.

1. A method for reducing sound track sibilance distortion duringplayback of an analog optical sound track, comprising the steps of: a)forming an image of said analog optical sound track; b) storing saidimage; c) representing part of said stored image as a spatial image; d)erosion filtering said spatial image to reduce image spread distortionof the optical sound track envelope; e) converting said filtered spatialimage to a signal representative of said sound track, said convertingcomprising: reviewing an audio quality of said signal representative ofsaid sound crack formed from said repeated steps, said reviewingincluding at least one of assessing sibilance distortion in said soundtrack, and assessing cross modulation distortion in said sound track;and, selecting one of no further erosion filtering and a differentamount of erosion filtering of said spatial image based on saidassessing; repeating steps c), d) and e) filter said stored image ofsaid analog optical sound track.
 2. The method of claim 1 comprising thestep of; processing said signal representative of said sound track toform a digital audio sound file signal as an output signal.
 3. A methodfor reducing audio distortion resulting from image spreading in ananalog optical sound track, comprising the steps of: a) forming an imageof said analog optical sound track with image spreading; b) storing saidimage; c) quantizing said stored image to have a first digital valuerepresenting audio modulation present on said analog optical sound trackand to have a second digital value representing said analog opticalsound track without said audio modulation; d) representing part of saidquantized stored image as a spatial image; e) selectively changing saidfirst digital value to said second digital value for a specific pixel ofsaid quantized stored image represented as a spatial image; and, f)converting said spatial image with said selectively changed pixel valueto a signal representative of said audio modulation present on saidsound track.
 4. The method of claim 3, wherein said steps e) and f) arerepeated for a different specific pixel of said quantized stored imageuntil every pixel of said spatial image has been selectively changed. 5.The method of claim 4, wherein said steps d) e) and f) are repeateduntil every pixel of said quantized stored image represented as spatialimages has been selectively changed.
 6. The method of claim 3, whereinsaid selectively changing step comprises; executing an algorithm todetermine whether said specific pixel is an edge pixel.
 7. The method ofclaim 3, wherein said selectively changing step comprises; performing alogical function to change said specific pixel located at an edge fromsaid first digital value to said second digital value.
 8. The method ofclaim 3, wherein said representing step comprises; stepping astructuring element over said spatial image and at an individual pixellocated by said structuring element executing an algorithm to determinechanging said first digital value to said second digital value.
 9. Themethod of claim 8, wherein said structuring element of said steppingstep includes coefficients that determine spatial image elements used insaid algorithm.
 10. The method of claim 3, wherein said representingstep comprises; stepping a structuring element over said spatial imageand at an location in said spatial image determined by said structuringelement executing an algorithm to determine changing a first pixel andan immediately adjacent pixel from said first digital value to saidsecond digital value.
 11. The method of claim 3, comprising the step ofassessing distortion present in said signal representative of said audiomodulation present on said sound track and selecting a differentstructuring element to effect changing from said first digital value tosaid second digital value at said first pixel and an immediatelyadjacent pixel, and repeating steps d), e) and f).